Radio Frequency (RF) front end circuits are used in wireless devices such as mobile phones, personal digital assistants, lap-top computers and other communication devices. The front end circuits are typically coupled to a transceiver chip (e.g., Bluetooth or ZigBee) in a wireless device. They increase the range of a wireless link by delivering increased output power during transmission along with low-pass filtering of harmonics while band-pass filtering during reception.
The front end circuits are often implemented as integrated modules. FIG. 1A illustrates a conventional RF front end circuit 100, which may be implemented as an integrated module interfacing with a transceiver chip 104 and an antenna 108. The front end circuit 100 includes a transformer 112 (balun) with its primary and secondary windings configured to provide a differential terminal 112D and a single-ended terminal 112S. The balun may be implemented by inductor-coupled printed or lumped-element components. During a transmit mode, the transformer 112 receives a differential RF signal at the differential terminal 112D from the transceiver chip 104 and converts the differential RF signal into a single-ended RF transmit signal at the single-ended terminal 112S. A single pole double throw (SPDT) switch 116 is coupled to the transformer 112. More specifically, the SPDT switch includes Ports 1-3, Port 1 being connected to the single-ended terminal 112S of the transformer 112 and Port 2 being connected to the input of a power amplifier 120. The internal connections among Ports 1-3 are controlled by a transmit/receive signal from the transceiver chip 104 (e.g., general purpose input-output (GPIO) signal) so that during the transmit mode Port 1 is connected to Port 2 and during the receive mode Port 1 is connected to Port 3. The single-ended RF transmit signal is routed by the SPDT switch 116 via Ports 1 and 2 to the power amplifier 120.
The output of the power amplifier 120 is coupled to a SPDT switch 124. More specifically, the SPDT switch 124 includes Ports 1-3, Port 1 being connected to a band pass filter 128 and Port 2 being connected to the output of the power amplifier 120. Port 3 of the SPDT switch 124 is connected to Port 3 of the SPDT switch 116. The power amplifier 120 amplifies the single-ended RF transmit signal and generates an amplified transmit signal in order to provide increased transmit power for enhancing the range of the wireless link. The amplified transmit signal is received at Port 2 of the SPDT switch 124. Responsive to a transmit/receive control signal from the transceiver chip 104, the internal connections of the SPDT switch 124 are configured so that Port 1 is connected to Port 2 during the transmit mode and Port 1 is connected to Port 3 during the receive mode. The SPDT switch 124 routes the amplified transmit signal to the band pass filter 128 via Ports 2 and 1. The band pass filter 128 substantially attenuates frequencies outside a selected pass band from the amplified transmit signal and generates a filtered transmit signal that is provided to the antenna 108. The antenna 108 converts the filtered transmit signal into electromagnetic waves for wireless transmission.
During the receive mode, a receive signal from the antenna 108 is filtered by the band pass filter 128. The filtered receive signal is received by the SPDT switch 128 at Port 1. Since during the receive mode, Ports 1 and 3 of the both the SPDT switches 124 and 116 are connected, the filtered receive signal is routed by the switches 124 and 116 to the single-ended terminal 112S of the transformer 112. The transformer 112 converts the filtered unbalanced receive signal into a differential receive signal, which is provided to the transceiver chip 104 via the differential terminal 112D.
FIG. 1B illustrates a conventional, enhanced sensitivity RF front end circuit 140. The front end circuit 140 includes a balun 142 having a differential terminal 142D and a single ended terminal 142S. The differential terminal 142D of the balun 142 is coupled to a transmit/receive port (RF_TX/RX) of a transceiver 144.
The front end circuit 140 includes a single pole double throw (SPDT) switch 146 coupled to the balun 142. The SPDT switch 146 includes Ports 1-3, Port 1 being connected to the single-ended terminal 142S of the balun 142, Port 2 being connected to the input terminal 148I of a power amplifier 148, and Port 3 being connected to the output terminal 150O of a low noise amplifier (LNA) 150. The internal connections among Ports 1-3 are controlled by a transmit/receive signal from the transceiver 144 (e.g., general purpose input-output (GPIO) signal) so that during the transmit mode Port 1 is connected to Port 2 and during the receive mode Port 1 is connected to Port 3.
During the transmit mode, the single-ended RF transmit signal is routed by the SPDT switch 146 via Ports 1 and 2 to the input terminal 148I of the power amplifier 148. The output terminal 148O of the power amplifier 148 is coupled to a SPDT switch 152. The SPDT switch 152 includes Ports 1-3, Port 1 being connected to a band pass filter 154, Port 2 being connected to the output terminal 148O of the power amplifier 148, and Port 3 being connected to the input terminal 150I of the LNA 150.
During the transmit mode, the power amplifier 148 amplifies the single-ended RF transmit signal and generates an amplified transmit signal. The amplified transmit signal is received at Port 2 of the SPDT switch 152. Responsive to the transmit/receive control signal from the transceiver 144, the internal connections of the SPDT switch 152 are configured so that Port 1 is connected to Port 2 during the transmit mode and Port 1 is connected to Port 3 during the receive mode. The SPDT switch 152 routes the amplified transmit signal to the band pass filter 154 via Ports 2 and 1. The band pass filter 154 substantially attenuates frequencies outside a selected pass band from the amplified transmit signal and generates a filtered transmit signal that is provided to the antenna 156.
During the receive mode, responsive to the transmit/receive control signal from the transceiver 144, the internal connections of the SPDT switch 152 are configured so that Ports 1 and 3 are connected. Likewise, during the receive mode, the internal connections of the SPDT switch 146 are configured so that Ports 1 and 3 are connected. Thus, it will be appreciated that a receive signal from the antenna 156 is filtered by the band pass filter 154, and the filtered receive signal is received at Port 1 of the switch 152. Since Port 1 is connected to Port 3 in the receive mode, the filtered receive signal is transferred via Port 3 to the input terminal 150I of the LNA 150. The LNA 150 amplifies the filtered receive signal to increase receiver sensitivity and generates an amplified receive signal at the output terminal 150O. The amplified receive signal is received at Port 3 of the switch 146. Since, Port 3 is connected to Port 1 in the receive mode, the amplified receive signal is transferred to the single-ended terminal 142S of the balun 142 via Port 1. The transformer 142 outputs a differential receive signal at the differential terminal 142D, which is provided to the transceiver 144.
FIG. 1C shows a conventional dual mode RF front end circuit 170 that may interface with a Bluetooth transceiver 172 and a WLAN transceiver 174. The Bluetooth transceiver 172 and the WLAN transceiver 174 operate in the same frequency band. The construction of the front end circuit 170 differs from that of the front end circuit 100 shown in FIG. 1 due to the fact that the front end circuit 170 features a first balun 176 adapted to interface with the Bluetooth transceiver 172 and a second balun 178 adapted to interface with the WLAN transceiver 174. A single pole triple throw (SP3T) switch 180 is controlled by a GPIO signal to either enable the WLAN transceiver 172 or the Bluetooth transceiver to transmit and/or receive. Two SPDT switches 182 and 184 are selectively controlled to route transmit signal through a power amplifier 186 during the transmit mode, but to remove the power amplifier 186 from the signal path during the receive mode. The operation of the conventional dual mode RF front end circuit 170 will be apparent to those skilled in the art and thus will not be described herein.
There are several disadvantages associated with existing enhanced sensitivity front end circuits. The front end circuits require two switches to operate, which increases cost and space requirement inside a module. The need for two switches also causes increased power loss during a receive mode. Also, the front end circuits require a power amplifier and a low noise amplifier, thus requiring increased space and additional cost.